staging: comedi: hwdrv_apci1500: introduce z8536_read()

The Z8536 CIO registers are indirectly read by writing the register offset to
the control register then reading the control register to get the value.

Introduce a helper function to read the Z8536 CIO registers.

The registers are read from "normal" code and in the interrupt handler,
spinlock the sequence to protect the indirect register access.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>